[Bug 682172] New: Avoid reading parts of file not used by pipeline

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=682172
  GStreamer | gst-plugins-good | git

           Summary: Avoid reading parts of file not used by pipeline
    Classification: Platform
           Product: GStreamer
           Version: git
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: dmj726 at sbcglobal.net
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


Currently Gstreamer will read all of a file from disk even if certain streams
are not needed and could be skipped.  This can be an issue when, for example,
one wants to process only the audio stream from a file containing both audio
and high-bitrate video, since performance will be bottlenecked by hard disk
throughput.
If elements like qtdemux could determine which streams aren't being used, avoid
reading data for unused streams, and instead only read the index and the
streams which are being used; this would hugely increase performance in those
use cases.

A good test uses footage from a Canon DSLR, which is commonly used for
professional video.  One 12 minute file is 4GB in size, of which less than
150MB is audio.  
The following pipeline takes approximately as long as simply reading the entire
file from disk. (about 40 seconds *uncached* on a Sandybridge laptop using the
file below)
time gst-launch-1.0 filesrc location="$INPUT_FILE" ! qtdemux ! audioconvert !
filesink location="OUTPUT_FILE"
When I profiled it, most time was spent in functions for reading from disk
(kernel -> system_call_fastpath -> sys_read), with CPU usage remaining low.

After extracting the audio only and placing it in another qt container, the
time required was dramatically reduced.

Example DLSR file can be found here (about 4GB).
http://uds-o.novacut.com/GWQ5M6GPPNVEY3SAGUORKMLUAMQGD6O4.mov

We should figure out a way to signal this behavior properly for elements like
qtdemux.

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